This invention relates generally to a gas turbine apparatus, and more particularly to a gas turbine apparatus of the type which is installed in power generating facilities, and uses city gas or liquefied natural gas as a fuel. The present invention also relates to a method for operating such a gas turbine apparatus.
A gas turbine apparatus has an operational mode called "load shutdown" in which, in the event of abnormality in a generator serving as a load, the load is disconnected to thereby protect the gas turbine. In this mode, the load of the gas turbine is abruptly changed from 100% to 0%, and this has heretofore been achieved by abruptly closing a fuel flow control valve. At this time, a rotational speed of the gas turbine is controlled by a gas turbine control device to be within a certain range.
In the type of gas turbine apparatus using city gas or liquefied natural gas as a fuel, a centrifugal compressor is used to supply the fuel to a combustor of the gas turbine apparatus. In the event of the above-described load shutdown, the rotational speed of this centrifugal compressor transiently increases, or a discharge pressure thereof increases. When this discharge pressure exceeds an allowable upper limit value, the fuel, such as city gas or liquefied natural gas excessively flows into the combustor, thereby causing the gas turbine to rotate at an unduly high speed. In order to prevent such a situation, there has heretofore been used a method in which the flow rate of the gas, handled by the centrifugal compressor for feeding the fuel under pressure, is measured, and when the flow rate decreases, a valve (hereinafter often referred to as "bypass valve") provided in a flow passage is opened.
By the way, when the flow rate of the handled gas is reduced, the pressure and the flow rate suddenly fluctuate in the centrifugal compressor to cause a surging phenomenon in which vibrations increase. In order to prevent this phenomenon, there is widely known a method, as described above, in which the flow rate of the gas is measured, and if the flow rate is lower than the predetermined value, the rate of flow through the centrifugal compressor is increased to prevent the surging of the centrifugal compressor, and also the bypass valve or a discharge valve is opened, thereby controlling the rate of flow to be fed to the gas turbine to a predetermined level. Such surging prevention method is disclosed, for example, in "JSME Mechanical Engineering Handbook, Revised 6th Edition" compiled by the Japan Society of Mechanical Engineers (1985; Vol. 10, pages 10-33).
The above conventional control method, in which the rate of flow of the handled gas is measured, and when this flow rate decreases, the bypass valve is opened, suffers from a drawback that since the bypass valve used for controlling the flow rate of the fuel of the gas turbine is closed instantaneously (about 0.1 second), the discharge pressure of the centrifugal compressor exceeds the above upper limit value.